netfilter: ctnetlink: acquire ct->lock before operating nf_ct_seqadj

We should acquire the ct->lock before accessing or modifying the
nf_ct_seqadj, as another CPU may modify the nf_ct_seqadj at the same
time during its packet proccessing.

Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>